Technical Field
The present invention relates to a material for an organic electroluminescence device and an organic electroluminescence device prepared by using the same.
Background Art
An organic electroluminescence device (hereinafter “electroluminescence” shall be abbreviated as EL) is a spontaneous light emitting device making use of the principle that a fluorescent substance or a phosphorescent substance emits light by recombination energy of holes injected from an anode and electrons injected from a cathode by applying an electric field. Since a laminate type organic EL device operated at a low voltage was reported, researches on organic EL devices comprising organic materials as structural materials have actively been carried out. In the above laminate type device, tris(8-quinolinolate)aluminum is used for the light emitting layer, and a tetraphenyldiamine derivative is used for the hole transporting layer. The advantages of the laminate structure include an elevation in an efficiency of injecting holes into a light emitting layer, a rise in a production efficiency of excitons produced by blocking electrons injected from a cathode to recombine them and shutting up of excitons produced in a light emitting layer. As shown in the above example, a two-layer type comprising a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layer type comprising a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known as the device structures of an organic EL device. In the above laminate type structural devices, device structures and forming methods are studied in order to enhance a recombination efficiency of holes and electrons injected.
Known as light emitting materials for an organic EL device are light emitting materials such as chelate complexes such as a tris(8-quinolinolate)aluminum complex and the like, coumarin derivatives, tetraphenylbutadiene derivatives, distyrylarylene derivatives, oxadiazole derivatives and the like. It is reported that light emission of a blue color to a red color in a visible region is obtained from the above light emitting materials, and color display devices are materialized.
Fluorescent materials which emit light by a singlet exciton have so far been used as light emitting materials for an organic EL device. In recent years, it is proposed to make use of phosphorescence luminescence materials which emit light by a triplet exciton in addition to fluorescent materials (for example, non-patent documents 1 and 2). It is considered that when an electron is recombined with a hole in an organic EL device, singlet excitons and triplet excitons are formed in a proportion of 1:3 due to a difference thereof in a spin multiplicity, and therefore an organic EL device prepared by using a phosphorescence luminescence material can achieve a luminous efficiency which is larger by three to four times as compared with that of an organic EL device prepared by using only a fluorescent material. In blue color phosphorescent emission, however, it is difficult to achieve the high efficiency and the long lifetime, and a host material which achieves them is desired to be developed.
A compound to which two carbazole skeletons are bonded via linkage groups is proposed in a patent document 1. A compound in which two carbazole skeletons are bonded to one dibenzofuran skeleton or dibenzothiophene skeleton is proposed in a patent document 2 (for example, compounds 23 and 24). A compound in which two carbazole skeletons are bonded to one dibenzofuran skeleton is proposed in a patent document 3 (for example, compound 43).
However, compounds which have two carbazole skeletons and to which a dibenzofuran skeleton or a dibenzothiophene skeleton is bonded at sites of N in the respective skeletons, if necessary, via linkage groups are not described in the above documents.
Further, the compounds described in the patent documents 1 to 3 were unsatisfactory in an efficiency and a lifetime in blue color phosphorescent emission.    Patent document 1: WO2007/108459    Patent document 2: WO2007/119816    Patent document 3: WO2007/077810    Non-patent document 1: Applied Physics Letters Vol. 74, No. 3, pp. 442 to 444    Non-patent document 2: Applied Physics Letters Vol. 75, No. 1, pp. 4 to 6